A consistent phenomenological model for natural organic matter linked migration of Tc(IV), Cm(III), Np(IV), Pu(III/IV) and Pa(V) in the Boom Clay

Long-term, sequential column migration experiments with Tc(IV), Cm(III), Np(IV), Pu(III/IV) and Pa(V) through Boom Clay cores were used to interpret the Organic Matter (OM) linked radionuclide (RN) transport.Based on observed processes, a consistent phenomenological model is proposed to describe the transport behaviour of the RN–OM linked species. The phenomenological model considers the radionuclide to be transported as an organic matter complex/colloid that slowly dissociates, and both the RN–OM and the RN sorb to the solid phase.Based on independently determined OM transport parameters and radionuclide–OM interaction constants, the model is well constrained and requires only 3 fitting parameters: the decomplexation constant, kdecomp, and the retardation factors for both the RN and the RNOM species, RRN and RRNOM. The resulting fitted dissociation kinetics are found to be in a narrow range (0.3–1.7 × 10−6 s−1), despite the difference in aqueous speciation. The fitted radionuclide retardation factors were found to be consistent with independent determined sorption data, which provides confidence in the conceptual model. The dissociation kinetics govern the transfer of RN mobilised by OM to RN sorbed on the solid phase and the kinetic decomplexation leads to an appreciable retardation of the colloid facilitated transport. This conceptual model is an important step forward in the description of OM linked RN transport in Boom Clay and it can be directly used for safety assessment analysis: (i) it is process based, (ii) the model is easy to implement without oversimplification and (iii) elements with a similar OM association behaviour can be described in the same way and the input parameters fall within a narrow range.